Low-temperature liquefied gases such as LNG (Liquefied Natural Gas) are stored, for example, in cylinder-shaped double-shell tanks having an internal tank made of metal and an outer tank made of concrete. The following process is generally followed when building this type of double-shell tank. Concrete outer tank shell plates are formed in a plurality of stages in the height direction from the base, an outer tank roof is then formed inside these and is lifted up. Then, internal tank shell plates are formed underneath the outer tank roof in a plurality of stages in the height direction in the same way as the outer tank shell plates.
In this conventional building method, it is necessary for a large-size forming frame to be set up inside the outer tank side until the outer tank shell plates have been built up to a certain height. Because of this, work to form the tank internal structure on the inside of the outer tank shell plates cannot proceed. For example, an annular plate (i.e., the previously mentioned internal structure) that is used to join together the internal tank shell plates and the bottom plate is positioned inside the tank, however, the task of positioning this annular plate cannot be performed until the outer tank shell plates have been built up to 3 or 4 levels.
In contrast, in Patent Document 1, a method is disclosed in which a precast concrete forming frame that is equipped with an outer tank liner, which is formed by integrating an outer tank liner plate with precast concrete, is used. According to this method, by setting up this precast concrete forming frame equipped with the outer tank liner on a base, and using it as a forming frame for pouring the concrete, the building of the outer tank lower portion can be performed concurrently with the forming of the tank internal structure.